Medical ultrasound device with temperature detection at distal end

ABSTRACT

A medical ultrasound device is disclosed. The device comprises an elongated body having a proximal end, a distal end ( 10 ) and a distal end region ( 1 ). One or more ultrasound transducers ( 4 ) for generating acoustic radiation are positioned in the distal end region, inside the elongated body. A transmission element ( 5 ) which is substantially transparent to acoustic radiation is positioned in the radiation path of the acoustic radiation, and a controller unit is operatively connected to the ultrasound transducer. The controller unit detects the acoustic path length through the transmission element and determines the temperature at the distal end from the detected acoustic path length. In an embodiment, the medical device is an ultrasound RF ablation catheter.

FIELD OF THE INVENTION

The invention relates to a medical ultrasound device, such as a probe orcatheter-based device. In particular the invention relates to suchdevices capable of detecting the temperature at the distal end of thedevice.

BACKGROUND OF THE INVENTION

Catheter-based surgery is advantageously used in various connections totreat body organs with minimal incision size and clearance of theorgans. As an example, cardiac arrhythmias may be treated by variouscatheter-based ablation techniques to destroy arrhythmogenic parts ofthe cardiac tissue. Specifically, radio-frequency (RF) ablation, highintensity focused ultrasound (HIFU) or cryo-ablations of the tissue arecommonly used.

In connection with the ablation process of cardiac tissue, it iscommonly used to monitor the temperature of the probe during theablation process. Since the probe is in close proximity with the tissueduring the ablation process, the probe temperature reflects the tissuetemperature. In some devices of the prior art, the ablation profile maybe controlled by the temperature, and direct thermal feedback may beused to titrate the ablation energy.

The US patent application no. 2006/0030844 A1 discloses to use atransparent electrode suitable for radiofrequency (RF) ablation oftissue. It is disclosed to cover a transparent material with aconductive coating so that the conductive coating is capable ofdelivering RF energy to the tissue, while the combined system oftransparent material and coating is transparent to radiation fromvarious imaging modalities. Different surface temperature means formeasuring the temperature are disclosed. For example, it is disclosed toplace a thermocouple on the electrode surface.

The placement of thermocouple on the electrode surface however puts thethermocouple in the field of view. While this may be acceptable for someapplication, this may not be the case for all applications. Moreover,there is still a need in the art for alternative or improved temperaturesensing solutions, suitable for use in connection with catheter-basedsurgery.

SUMMARY OF THE INVENTION

The inventors of the present invention have realized thatthermocouple-based temperature sensing solutions may not be suitable foruse in connection with medical devices comprising integrated ultrasoundmonitoring in the forward looking geometry, since the positioning of thethermocouple may be in the field of view of the acoustic radiation.Consequently it would be advantageous to achieve a temperature sensingsolution which is suitable for integration with medical devicescomprising integrated ultrasound transducers. In general, the inventionpreferably seeks to mitigate, alleviate or eliminate one or more of theabove mentioned disadvantages singly or in any combination. Inparticular, it may be seen as an object of the present invention toprovide a medical device that solves the above mentioned problems, orother problems, of the prior art.

To better address one or more of these concerns, in a first aspect ofthe invention a medical ultrasound device is presented that comprising:

-   -   an elongated body having a proximal end, a distal end, a distal        end region and a length axis along the elongation;    -   one or more ultrasound transducers for generating acoustic        radiation, the one or more ultrasound transducers being        positioned in the distal end region, inside the elongated body;    -   a transmission element positioned in the radiation path of the        acoustic radiation, wherein the transmission element is        substantially transparent to acoustic radiation;    -   a controller unit operatively connected to the ultrasound        transducer;

wherein the controller unit detects the acoustic path length through thetransmission element and determines the temperature at the distal endfrom the detected acoustic path length.

The invention provides a medical device, such as a catheter or probe,with integrated ultrasound facilities, where the ultrasound radiationcan be used for general purposes, as well as for generating a measure oftemperature of the transmission element. Since during use, thetransmission element will be in close contact with tissue underinvestigation or treatment, this temperature will be the same as, orclose to, the temperature of the tissue. By measuring the temperature ofthe transmission element, the temperature at the distal end of themedical ultrasound device, and hence the temperature of the tissue underinvestigation or treatment, can be determined. In an advantageousembodiment, the one or more ultrasound transducers are capable ofgenerating acoustic radiation suitable for monitoring a region ofinterest simultaneously with, concurrently with or together withdetecting the acoustic path length through the transmission element. Bybasing the detection of the temperature on ultrasound radiation and pathlength detection through the transmission element, key elements toperform the temperature detection are elements which also may be usedfor other purposes, and a separate sensor is not needed. The ultrasoundtransducer(s) may be used for monitoring purposes and a transmissionelement is always needed in order to couple the acoustic radiation outof the medical device. A compact and cost-effective medical device istherefore provided. Moreover, in devices with a forward lookinggeometry, a temperature sensor may be provided which does not shadow theacoustic radiation.

In the context of the present invention, monitoring is to be construedbroadly. It includes both 1D monitoring, i.e. detecting reflectedintensities along the line of sight as well as 2D imaging where an arrayof transducers are applied to generate a 2D image. In principle also 3Dimaging and time resolved imaging may be obtained. In catheter-basedmonitoring, it is normal to use 1D or 2D monitoring due to spaceconstraints in the distal end region, i.e. in the tip region.

In general, the transmission element should be substantially transparentto acoustic radiation. A number of materials, including various polymermaterials, fulfill this. In general any material can be used, as long asthe transparency is sufficient to enable clinical use as well as toenable detection of the acoustic path length through the element. Inparticular, a material with a transparency to acoustic radiation above50% may be used, such as above 60%, 70%, 80%, 90%, or even above 95%.

The acoustic path length is detected based on detecting reflectedacoustic radiation from the transmission element. In an advantageousembodiment, the acoustic path length is detected based on detectingreflected acoustic radiation from a surface of the backside of thetransmission element and a surface of the front-side of the transmissionelement, the acoustic path length may be detected based on detecting aseparation of reflection peaks obtained from the surface of the backsideof the transmission element and the surface of the front-side of thetransmission element. The detection of the acoustic path length may bebased on a detection of the time of flight, and changes in time offlight, of radiation emitted from the transducer, reflected from asurface of the transmission element, and detected again by thetransducer.

In an advantageous embodiment, the polymer-based body has a change ofvelocity of the acoustic radiation larger than 0.1% per degree Celsiusor larger, such as 0.25% per degree Celsius or even larger.

In an advantageous embodiment, the distal end region further comprisesfluid channels which allow delivery of fluid through the elongated bodyto the distal end region. Typically, saline fluid may be pumped from areservoir placed at the proximal end to irrigate the area underinvestigation or treatment.

In an advantageous embodiment, the temperature at the distal end isdetermined based on a look-up table or a functional relationship betweena parameter related to the acoustic path length and the temperature atthe distal end. Such relationships can be deduced from laboratoryexperiments or calibration routines. Use of look-up tables or functionalrelationships facilitates fast and flexible ways of correlating themeasured path length to the temperature during clinical use.

Advantageously, the transmission element comprises a treatment modalityfor treatment of body tissue. In an embodiment, the treatment modalityis ablation, such as radiofrequency (RF) ablation.

In an embodiment the ablation is performed by use of an electrodesupported by the transmission element. The electrode may be providedsuch that the acoustic radiation is substantially unaffected by thepresence of the electrode. In an embodiment, the electrode is in theform of a thin layer sufficiently thin to be substantially transparentto acoustic radiation. Acoustic radiation will be transmittedsubstantially unaffected by the presence of a metal layer with athickness below 500 nanometers, such as below 250 nanometers, such aswith a thickness of 150 nanometers. In other embodiments, the electrodemay be in the form of a mesh or other open structures. An electrode inthe form of a mesh, with a central aperture or even in the form of aband or ring, may allow radiation to pass, and still be able to work asan RF-electrode.

In a second aspect of the invention, a method of operating a medicaldevice is presented. A medical device in accordance with the firstaspect of the invention is operated by steps which comprise:

-   -   generate acoustic radiation by operating the one or more        transducers in a generation mode;    -   detect reflected acoustic radiation by operating the one or more        transducers in a detection mode;    -   from the reflected acoustic radiation detect the acoustic path        length of the acoustic radiation through the transmission        element;    -   determine the temperature at the distal end from the detected        acoustic path length.

In a third aspect of the invention, a computer program product ispresented that is adapted to enable a computer system comprising atleast one computer having data storage means associated therewith tooperate a medical device according to according to the first aspects ofthe invention or to carry out the steps of the second aspect of theinvention.

In general the various aspects of the invention may be combined andcoupled in any way possible within the scope of the invention. These andother aspects, features and/or advantages of the invention will beapparent from and elucidated with reference to the embodiments describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 schematically illustrates the distal end region of an ablationcatheter-based probe;

FIG. 2 schematically illustrates an ablation electrode supported by atransmission element;

FIG. 3 illustrates a screen shot of an M-mode ultrasound image ofcardiac ablation in a sheep heart;

FIG. 4 illustrates a zoom made of the first order TPX/Pt reflection peakof the M-mode image of FIG. 3;

FIG. 5 shows a graph of peak separation as a function of time;

FIG. 6 illustrates a graph correlating the peak separation, the speed ofsound and the temperature;

FIG. 7 further illustrates peak separations as a function oftemperature;

FIG. 8 illustrates a flow diagram of steps performed in connectionoperating a medical device; and

FIG. 9 schematically illustrates a medical device connected to acontroller unit and in connection with a computer program product.

DESCRIPTION OF EMBODIMENTS

The present invention is disclosed in connection with a RF ablationcatheter comprising a monitoring system in accordance with embodimentsof the present invention. It is however to be understood that, whilesuch an application is advantageous, the invention is not limited tothis. In fact, the medical device may be applied in connection with anydevice which uses ultrasound transducers and which supports a structuralconfiguration which enables to detect an acoustic path length through atransmission element.

FIG. 1 schematically illustrates the distal end region 1 of an ablationcatheter-based probe, hereafter also simply referred to as a catheter.The catheter comprises an elongated body 3, a proximal end (not shown),a distal end 10 and a distal end region 1. A length axis 9 runs alongthe elongation of the elongated body. The distal end region 1 is theextended end section of the elongated body 3 abutting the distal enditself 10. The catheter may at the proximal end comprise a controllerunit or connection for a controller unit (cf. FIG. 9). The ultrasoundtransducer 4 is housed in the distal end region, where it is fixed bysuitable means 6. The catheter comprises a transmission element 5positioned in the radiation path of the acoustic radiation. Thetransmission element may be used as a transmission window for couplingthe acoustic radiation out of the medical device. The transmissionelement has a backside generally facing the ultrasound transducer and anopposite facing front-side. The transmission element is substantiallytransparent to acoustic radiation, so that radiation generated by theultrasound transducer will be transmitted through the transmissionelement to interact with tissue 2 under investigation or treatment. Inan embodiment, the acoustic radiation is emitted along the length axis9.

As is illustrated in FIG. 1, the distal end region may further comprisefluid channels 7 which allow delivery of fluid through the elongatedbody to the distal end region so as to irrigate the treatment siteduring treatment if this is necessary or desirable, typically by use ofsaline fluid. The fluid channels may be holes into the side of the tubeas in the illustrated embodiment, or made by other suitable means.

In an embodiment the device may e.g. be an ultrasound catheter with anintegrated ablation electrode. The ultrasound catheter supportsmonitoring of tissue properties by operating the ultrasound transducerin a monitoring mode, where ultrasound pulses are emitted and thereflected radiation is detected in order to generate an ultrasound imageor scan. Operating an ultrasound transducer for detecting reflectedradiation is known to the skilled person.

The elongated body may be of a flexible material, such as a suitablepolymer material for use in connection with a medical device. Suchmaterials are known to the skilled person. A flexible device is therebyobtained. Alternatively may the elongated body be made of a rigidmaterial, such as surgical steel or other suitable materials as areknown to the skilled person. A rigid device may e.g. be implemented as aneedle device.

FIG. 2 schematically illustrates an ablation electrode 20 supported by atransmission element 5. The transmission element has a backside 21 and afront side 22. The ablation electrode may be formed by a thin conductinglayer supported by the transmission element. In an embodiment, thetransmission element comprises a polymer-based body and a conductinglayer. The polymer-based body may be of the material poly-methylpentene(TPX) which is commonly used in connection with ultrasound, whereas theconducting layer may be a metallic layer, such as a platinum layer.Suitable thicknesses may be a few hundred micrometers thick TPXsupporting a few hundred nanometer thick platinum layer, such as a 250micrometer thick TPX element, supporting a 150 nanometer thick platinumlayer. The thickness of the TPX element is the thickness at the centralregion. Other materials may also be used, as long as they aresufficiently transparent to acoustic radiation. The transmission elementand supported electrode are illustrated in a rounded configuration whichis the clinically relevant shape. In general any shape may be used.

FIG. 3 illustrates a screen shot of an M-mode ultrasound image ofcardiac ablation in a sheep heart as generated by an ablation catheterof the type schematically illustrated in FIG. 1. The vertical axis showsthe distance from the transducer. The distance is given in pixels whichcan be converted into time or depth. The horizontal axis illustratestime, again given in pixels (increments of 20 pixels equals 1 second).The image shows the strong primary reflection 30 from the TPX/Ptablation electrode, and in addition 2nd and 3rd order reflection peaks31, 32.

FIG. 4 illustrates a zoom made of the first TPX/Pt reflection peak 30,as indicated with reference numeral 33 on FIG. 3. In FIG. 4, it can beseen that the two peaks (maxima indicated by reference numerals 40, 41)are observed. The positions of these reflections are related to thetime-of-flight of the ultrasound signal, and therefore the acoustic pathlength through the transmission element. The maxima of the two peaks areobserved to be relatively constant with respect to time in the firsthalf of the image, however as can be seen during the period indicatedwith reference numeral 42 where the ablation process is running, thedistance 43, 44 between the two peaks increases. The first peak 40corresponds to the transition of the acoustic radiation into thetransmission element, and the second peak 41 corresponds to thetransition of the acoustic radiation out of the transmission element. Inthe area between the two peaks, the ultrasound radiation is propagatinginside the transmission element. Due to the ablative process, thetemperature of the ablation electrode and the tissue increases and as aresult, the acoustical path length through the transparent ablationelectrode increases too. By monitoring the positioning of the two peaks,the acoustic path length can be monitored. From analysis of themonitored data, it is possible to obtain sub-pixel resolution. The mainphysical effect which gives rise to the changes in the acoustical pathlength is the change of the speed of sound in dependence upon thetemperature changes. It can be mentioned that the material expansion ofeither the electrode or the transmission element over the relevanttemperature ranges is nearly negligible. As the temperature rises, thespeed of sound decreases, resulting in an increases acoustical pathlength, which is seen as an increase in the distance 43, 44 between thetwo peaks.

FIG. 5 shows a graph of the peak separation 43, 44 as a function of timein the ablation period as indicated with reference numeral 42 in FIG. 4.The vertical axis is peak separation in pixels and the horizontal axisis time in seconds. The graph shows measuring points 50 as well as acalculated line 51 of the expected thermal effect. The calculation wasobtained by assuming 4 mm thick cardiac tissue, cold surfaces and a 6 mmdiameter ablation catheter. The vertical axis includes only a singlefitting parameter in the form of the product of the ablation power andthermal conductivity. The horizontal axis does not contain fittingparameters. As can be seen, during the ablation process, the acousticalpath length through the transmission element clearly increases.Subsequently, at the end of the ablation (at time=60 sec.) a rapidcooling is observed. The final jump at time=70 sec. is due to removal ofthe device from the heart wall.

FIG. 6 illustrates a graph correlating the peak separation (leftvertical axis), the speed of sound (right vertical axis) and thetemperature in degree Celsius (horizontal axis). The measurement pointsare shown as solid bullets 60 (a line is drawn through the points toguide the eye), moreover, a line 61 is shown indicating 0.25% expansionper ° C. of the acoustic path length for comparison to the data. As canbe seen the catheter is capable of accurately determining thetemperature at the location of the point of contact between the ablationelectrode and the tissue, which is the clinically interesting point.

FIG. 7 further illustrates peak separations as a function oftemperature. FIG. 7 illustrates a laboratory experiment, where theacoustical path length between the two peaks was measured for a medicaldevice with the distal end region submerged in a water bath for a seriesof constant temperatures. A line 70 is shown which indicate 0.25%expansion per ° C. of the acoustic path length for comparison to thedata. Point connected by the line with reference numeral 71 connect datapoints obtained during temperature rise 72, whereas point connected bythe line with reference numeral 73 connect data points obtained duringtemperature decent 74. As can be seen, thermal resolution is of theorder of 1° C. within the range of clinical relevant temperatures.

In a situation of use, the temperature at the distal end may bedetermined based on a look-up table or a functional relationship betweena parameter related to the acoustic path length and the temperature atthe distal end, e.g. as deduced from a measurement as presented in FIG.7. Look-up table, functional relationships etc. may be stored by andcomputed in the controller unit or a computing unit in or connected tothe controller unit.

FIG. 8 illustrates a flow diagram of some of the steps which may beperformed in order to operate a medical device in accordance withembodiments of the present invention. Firstly, the medical device may bepositioned 80 in the region of interest, for example in close proximityof cardiac tissue to undergo ablation treatment. The transducers areoperated to generate 81 acoustic radiation and to detect 82 thereflected acoustic radiation. The transducers may be operatedcontinuously 83 during the investigation and treatment. The reflectedacoustic radiation is detected in order to monitor 84 the region ofinterest during the procedure, and from the reflected acoustic radiationalso the acoustic path length is deduced to determine the temperature 85at the distal end. Simultaneously with the monitoring and thetemperature detection, the treatment modality may be operated 86 inorder to perform medical treatment. For example, the tissue undertreatment may undergo ablation.

FIG. 9 schematically illustrates a medical device connected to acontroller unit and in connection with a computer program product. Themedical device comprises a catheter in accordance with embodiments ofthe present invention. The catheter comprises an elongated body 3 havinga proximal end 90, a distal end 10, a distal end region 1 and a lengthaxis 9 along the elongation. Moreover, the catheter comprises one ormore ultrasound transducers positioned in the distal end region and atransmission element 5 positioned at the extremity of the elongated bodyto couple acoustic radiation in and out of the catheter.

The catheter is at the proximal end 90 connected to a controller unit91, such as a dedicated purpose or general purpose computing unit forcontrol of at least the ultrasound transducer(s) and for dealing withthe signal treatment and extraction of detection results. To this end,the detection of the acoustic path length through the transmissionelement and the determination of the temperature at the distal end arecontrolled by the controller unit 91.

The controller unit may implement a computer system 92, such as adedicated purpose or general purpose computing unit for controlling thedevice. The computer system may comprise storage means 93 for storingdata which may be needed to operate the medical device or to store anyacquired data, or for any other purpose where storage of data isdesired. The computer system may be adapted to receive instructions froma computer program product 94 in order to operate the device. Thecomputer program product may be comprised in a data carrier asillustrated in the Figure, however once loaded into the computer systemit may be stored by, and run from, the storage means 93.

In the foregoing, simultaneous operation of the monitoring, the ablationand the temperature sensing have been described. While it is anadvantage of embodiments of the present invention that such simultaneousoperation is feasible, also interleaved operation of one or more of theoperation modalities is possible if this is desired.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims. In the claims,the word “comprising” does not exclude other elements or steps, and theindefinite article “a” or “an” does not exclude a plurality. A singleprocessor or other unit may fulfill the functions of several itemsrecited in the claims. The mere fact that certain measures are recitedin mutually different dependent claims does not indicate that acombination of these measured cannot be used to advantage. A computerprogram may be stored/distributed on a suitable medium, such as anoptical storage medium or a solid-state medium supplied together with oras part of other hardware, but may also be distributed in other forms,such as via the Internet or other wired or wireless telecommunicationsystems. Any reference signs in the claims should not be construed aslimiting the scope.

1. A medical ultrasound device comprising: an elongated body (3) havinga proximal end (90), a distal end (10), a distal end region (1) and alength axis (9) along the elongation; f one or more ultrasoundtransducers (4) for generating acoustic radiation, the one or moreultrasound transducers being positioned in the distal end region, insidethe elongated body; a transmission element (5) positioned in theradiation path of the acoustic radiation, wherein the transmissionelement is substantially transparent to acoustic radiation; a controllerunit (91) operatively connected to the ultrasound transducer; whereinthe controller unit detects the acoustic path length through thetransmission element and determines the temperature at the distal endfrom the detected acoustic path length.
 2. The device according to claim1, wherein the transmission element is having a backside (21) generallyfacing the ultrasound transducer and an opposite facing front-side (22),and wherein the temperature at the distal end is determined based ondetecting a separation of reflection peaks (40, 41) from a surface ofthe backside of the transmission element and a surface of the front-sideof the transmission element.
 3. The device according to claim 1, whereinthe distal end region further comprises fluid channels (7) which allowdelivery of fluid through the elongated body to the distal end region.4. The device according to claim 1, wherein the one or more ultrasoundtransducers are capable of generating acoustic radiation suitable formonitoring a region of interest and for detecting the acoustic pathlength through the transmission element.
 5. The device according toclaim 1, wherein the temperature at the distal end is determined basedon a look-up table or a functional relationship between a parameterrelated to the acoustic path length and the temperature at the distalend.
 6. The device according to claim 1, wherein the transmissionelement comprises a polymer-based body which is substantiallytransparent to acoustic radiation, the polymer-based body having achange of velocity of the acoustic radiation larger than 0.1% per degreeCelsius.
 7. The device according to claim 1, wherein the transmissionelement comprises a polymer-based body which is substantiallytransparent to acoustic radiation, covered with an electrode (20)substantially transparent to acoustic radiation.
 8. The device accordingto claim 1, wherein the transmission element comprises a treatmentmodality for treatment of body tissue.
 9. The device according to claim1, wherein the device is an ultrasound catheter with an integratedablation electrode, wherein the transmission element comprises theintegrated ablation electrode.
 10. Method of operating a medical device,the device comprises: an elongated body (3) having a proximal end (90),a distal end (10), a distal end region (1) and a length axis (9) alongthe elongation; one or more ultrasound transducers (4) for generatingacoustic radiation, the one or more ultrasound transducers beingpositioned in the distal end region, inside the elongated body; atransmission element (5) positioned in the radiation path of theacoustic radiation, wherein the transmission element is substantiallytransparent to acoustic radiation; a controller unit operativelyconnected to the ultrasound transducer; wherein the method comprises:generate (81) acoustic radiation by operating the one or moretransducers in a generation mode; detect (82) reflected acousticradiation by operating the one or more transducers in a detection mode;from the reflected acoustic radiation detect the acoustic path length ofthe acoustic radiation through the transmission element; determine (85)the temperature at the distal end from the detected acoustic pathlength.
 11. A computer program product (94) being adapted to enable acomputer system (92) comprising at least one computer having datastorage means (93) associated therewith to operate a medical deviceaccording to claim 1.